I. Field of Invention
This invention relates to the production of ammonium phosphate. In particular, this invention relates to a process for making ammonium phosphates such as mono- and di-ammonium phosphates that are substantially free of arsenic from crude phosphoric acid.
II. Description of the Prior Art
For many commercial applications today, it is desirable to prepare ammonium phosphates from phosphoric acid. However, the cost involved when starting with highly pure grades of phosphoric acids (e.g., thermal grade acid) is normally too high to be economically advantageous. Therefore, it is necessary to explore the possibilities of employing cheaper phosphoric acid (e.g., wet process acid) to make ammonium phosphates. However, the employment of these less expensive grades of phosphoric acid may result in the ammonium phosphate products containing more impurities. In many instances these impurities are objectionable. For example, ammonium phosphate containing dissolved arsenic such as elemental arsenic, salts containing arsenic ions, chemical complexes containing arsenic and the like cannot be used for food or industrial grade applications. Instead such arsenic-containing ammonium phosphates are limited to less commercially attractive uses such as fertilizers and the like. Thus, because of this arsenic impurity problem, effective usage of the cruder grades of phosphoric acid as a starting material for making ammonium phosphate may be hindered. A need exists in the art to develop a process for making ammonium phosphate products having relatively low levels of arsenic from these less expensive grades of phosphoric acid. The present invention provides such a process.
The usual prior art process for making ammonium phosphate products from relatively pure phosphoric acid, as illustrated in U.S. Pat. No. 3,388,966, issued on June 18, 1968 to R. A. MacDonald, is to (1) ammoniate the phosphoric acid to form ammonium phosphate products and insoluble matter, (2) filter off the undesirable insoluble matter, (3) crystallize the ammonium phosphate products from the mother liquor and (4) recover these crystals. While this process is acceptable for producing arsenic-free ammonium phosphate products from relatively pure phosphoric acid, it cannot be used to produce arsenic-free ammonium phosphate from phosphoric acid containing large amounts of dissolved arsenic. Instead, this arsenic may co-crystallize with the ammonium phosphate products and prevent the latter's use as a food or industrial grade ammonium phosphates.
The prior art also teaches that dissolved arsenic can be removed from crude phosphoric acid by adding barium sulfide to precipitate the arsenic. See U.S. Pat. No. 2,044,940, issued on June 23, 1936 to I. L. Haag and W. R. Devor. One of the problems with this process is that an additional filtration step is also used to remove the resulting arsenic precipitates immediately after addition of the barium sulfide. This filtration step increases the cost of the overall ammonium phosphate process because extra solids-handling steps must now be included. Also, as explained below, the employment of barium sulfide may decrease the amount of P.sub.2 O.sub.5 in the ammonium phosphate product.